molding an object in the shape of another object
If I understand your question correctly,you are looking for the Shrinkwrap-Modifier. Add the modifier to the model you want to change and choose your "desired face" as the target. For further reading consult the manual. 

However there is no way to get more detail than your polycount allows. That means the Shrinkwrap-Modifier might produce artifacts if the polygon-density of your model is far less than that of your target.